Cast Iron Comprising Cobalt and Component

ABSTRACT

Cast iron alloys have application limits with regard to temperature. By means of the use of cobalt an optimal ferritic structure can be achieved such that with an alloy containing silicon 2.0-4.5 wt. %, cobalt 0.5-5 wt. %, carbon 2.5-4 wt. %, molybdenum≦1 wt. %, manganese≦0.25 wt. %, nickel≦0.3 wt. %, the remainder iron where the proportion of silicon cobalt and molybdenum is less than 7.5 wt. % the application limits are shifted to higher temperatures.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International ApplicationNo. PCT/EP2007/050057, filed Jan. 3, 2007 and claims the benefitthereof. The International Application claims the benefits of EuropeanPatent Office application No. 06000851.3 EP filed Jan. 16, 2006, both ofthe applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to an alloy, a cast iron comprising cobalt and acomponent thereof.

BACKGROUND OF INVENTION

The known cast iron alloys now employed (so-called GJS spherocastalloys) primarily use silicon and molybdenum to increase the creepstrength, scaling resistance and endurance strength. Over time, however,these elements lead to a significant decrease in the ductility.

Molybdenum furthermore exhibits a very high susceptibility tosegregation.

SUMMARY OF INVENTION

It is therefore an object of the invention to provide an alloy and acomponent, which overcome the aforementioned disadvantages and havebetter mechanical strengths over the service life.

The object is achieved by an alloy as claimed in an independent claimand e.g. by a component as claimed in a further independent claim.

Further advantageous measures are listed in the dependent claims, andthese may advantageously be combined with one another in any desiredway.

The invention consists in cobalt partially or fully replacingmolybdenum. The working limitations presented by the previous GJS alloycan therefore be overcome. The alloy according to the invention has highelongations for the application field in the temperature range of 450°C.-550° C., and has the following composition (in wt %):

silicon 2.0%-4.5% cobalt 0.5%-5% carbon 2.0%-4.5%, in particular2.5%-4%, molybdenum <1.5%, in particular ≦1.0%, manganese <0.5%, inparticular ≦0.25%, nickel <0.5%, in particular ≦0.3%, remainder iron.

Advantageously, the proportion of silicon, cobalt and molybdenum is lessthan 7.5 wt %.

Preferably, the proportion of cobalt in the alloy lies between 0.5 and1.5 wt % cobalt.

Advantageous mechanical values are achieved for the alloy respectivelywhen the cobalt content is 0.5 wt %, with 1 wt % cobalt, with 1.5 wt %cobalt and 2.0 wt % cobalt.

The alloy may contain further elements. Preferably, however, the alloyconsists of iron, silicon, cobalt and carbon.

Particular advantages are also achieved when the alloy consists of iron,silicon, cobalt, carbon and manganese.

Further advantages are obtained with an alloy which consists of iron,silicon, cobalt, carbon and optionally admixtures of molybdenum,manganese and/or nickel.

The alloy may optionally contain undesired impurities of at most

phosphorus 0.007 wt % sulfur 0.008 wt % magnesium 0.049 wt %.

Furthermore, there is preferably no chromium (Cr) in the alloy exceptfor the usual impurities.

Likewise, there is preferably no magnesium (Mg) in the alloy except forthe usual impurities.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be explained in more detail with theaid of the following figures, in which:

FIG. 1 shows a micrograph, FIG. 2 shows mechanical characteristics, FIG.3 shows a steam turbine, FIG. 4 shows a gas turbine.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows an almost optimal ferritic structure (etched) withspherical graphite made of an alloy with about 2 wt % cobalt:

carbon 3.67 wt %, molybdenum 2.41 wt %, manganese 0.029 wt %,  nickel1.94 wt %, iron remainder.

FIG. 2 shows the influence of cobalt on the mechanical properties of thealloys, which are listed in the following table (data in wt %).

cobalt 0 0.54 1.04 1.94 carbon 3.63 3.61 3.68 3.67 silicon 2.45 2.442.47 2.41 manganese 0.067 0.036 0.03 0.029 phosphorus 0.007 0.006 0.0070.007 Sulfur 0.009 0.006 0.008 0.008 Magnesium 0.044 0.04 0.05 0.049

The elongation at break R_(p02) increases from 271 N/mm² to 284 N/mm².

The tensile strength Rm increases from 403 N/mm² to 412 N/mm².The elongation at break A5 increases from 15.5% to 21.9%.Likewise, the necking at fracture Z increases from 13.8% to 29.5%.

Even small proportions of cobalt (0.5 wt % to 1.0 wt % or 1.0 wt % to1.5 wt %) improve the mechanical characteristics.

FIG. 3 shows a steam turbine 300, 303 having a turbine shaft 309extending along a rotation axis 306.

The steam turbine comprises a high-pressure turbine part 300 and amedium-pressure turbine part 303, each with an inner housing 312 and anouter housing 315 enclosing the latter. The high-pressure turbine part300 is, for example, configured in pot design. The medium-pressureturbine part 303 is, for example, configured to be twin-streamed. It islikewise possible for the medium-pressure turbine part 303 to beconfigured to be single-streamed.

A bearing 318 is arranged along the rotation axis 306 between thehigh-pressure turbine part 300 and the medium-pressure turbine part 303,the turbine shaft 309 comprising a bearing region 321 in the bearing318. The turbine shaft 309 is mounted on a further bearing 324 besidethe high-pressure turbine part 300. In the region of this bearing 324,the high-pressure turbine part 300 comprises a shaft seal 345. Theturbine shaft 309 is sealed relative to the outer housing 315 of themedium-pressure turbine part 303 by two further shaft seals 345. Betweena high-pressure steam intake region 348 and a steam outlet region 351,the turbine shaft 309 in the high-pressure turbine part 300 comprisesthe high-pressure rotor blading 357. With the associated rotor blades(not represented in detail), this high-pressure rotor blading 357constitutes a first blading region 360.

The medium-pressure turbine part 303 comprises a central steam intakeregion 333. Associated with the steam intake region 333, the turbineshaft 309 comprises a radially symmetric shaft shield 363, a coverplate, on the one hand to divide the steam flow into the two streams ofthe medium-pressure turbine part 303 and also to prevent direct contactof the hot steam with the turbine shaft 309. In the medium-pressureturbine part 303, the turbine shaft 309 comprises a second bladingregion 366 with the medium-pressure rotor blades 354. The hot steamflowing through the second blading region 366 flows from themedium-pressure turbine part 303 out of a discharge port 369 to alow-pressure turbine part (not shown) connected downstream.

The turbine shaft 309 is composed for example of two turbine shaft parts309 a and 309 b, which are connected firmly to one another in the regionof the bearing 318. Each turbine shaft part 309 a and 309 b comprises acooling line 372 formed as a central bore 372 a along the rotation axis306. The cooling line 372 is connected to the steam outlet region 351via a feed line 375 comprising a radial bore 375 a. In themedium-pressure turbine part 303, the coolant line 372 is connected to acavity (not shown) below the shaft shield. The feed lines 375 areconfigured as a radial bore 375 a, so that “cold” steam from thehigh-pressure turbine part 300 can flow into the central bore 372 a. Viathe discharge line 372 also formed in particular as a radially directedbore 375 a, the steam passes through the bearing region 321 into themedium-pressure turbine part 333 and there onto the lateral surface 330of the turbine shaft 309 in the steam intake region 333. The steamflowing through the cooling line is at a much lower temperature than thetemporarily superheated steam flowing into the steam intake region 333,so as to ensure effective cooling of the first rotor blade row 342 ofthe medium-pressure turbine part 303 and the lateral surface 330 in theregion of this rotor blade row 342.

FIG. 4 shows a gas turbine 100 by way of example in a partiallongitudinal section.

The gas turbine 100 internally comprises a rotor 103, which will also bereferred to as the turbine rotor, mounted so as to rotate about arotation axis 102 and having a shaft 101.

Successively along the rotor 103, there are an intake manifold 104, acompressor 105, an e.g. toroidal combustion chamber 110, in particular aring combustion chamber, having a plurality of burners 107 arrangedcoaxially, a turbine 108 and the exhaust manifold 109.

The ring combustion chamber 110 communicates with an e.g. annular hotgas channel 111. There, for example, four successively connected turbinestages 112 form the turbine 108.

Each turbine stage 112 is formed for example by two blade rings. As seenin the flow direction of a working medium 113, a guide vane row 115 isfollowed in the hot gas channel 111 by a row 125 formed by rotor blades120.

The guide vanes 130 are fastened on an inner housing 138 of a stator 143while the rotor blades 120 of a row 125 are fastened on the rotor 103,for example by means of a turbine disk 133.

Coupled to the rotor 103, there is a generator or a work engine (notshown).

During operation of the gas turbine 100, air 135 is taken in andcompressed by the compressor 105 through the intake manifold 104. Thecompressed air provided at the end of the compressor 105 on the turbineside is delivered to the burners 107 and mixed there with a fuel. Themixture is then burnt to form the working medium 113 in the combustionchamber 110. From there, the working medium 113 flows along the hot gaschannel 111 past the guide vanes 130 and the rotor blades 120. At therotor blades 120, the working medium 113 expands by imparting momentum,so that the rotor blades 120 drive the rotor 103 and the work enginecoupled to it.

During operation of the gas turbine 100, the components exposed to thehot working medium 113 experience thermal loads. Apart from the heatshield elements lining the ring combustion chamber 110, the guide vanes130 and rotor blades 120 of the first turbine stage 112, as seen in theflow direction of the working medium 113, are heated the most.

In order to withstand the temperatures prevailing there, they may becooled by means of a coolant.

Substrates of the components may likewise comprise a directionalstructure, i.e. they are monocrystalline (SX structure) or comprise onlylongitudinally directed grains (DS structure).

Iron-, nickel- or cobalt-based superalloys are for example used asmaterial for the components, in particular for the turbine blades 120,130 and components of the combustion chamber 110.

Such superalloys are known for example from EP 1 204 776 B1, EP 1 306454, EP 1 319 729 A1, WO 99/67435 or WO 00/44949; with respect to thechemical composition of the alloys, these documents are part of thedisclosure.

The blades 120, 130 may likewise have coatings against corrosion(MCrAlX; M is at least one element from the group iron (Fe), cobalt(Co), nickel (Ni), X is an active element and stands for yttrium (Y)and/or silicon, scandium (Sc) and/or at least one rare earth element, orhafnium). Such alloys are known from EP 0 486 489 B1, EP 0 786 017 B1,EP 0 412 397 B1 or EP 1 306 454 A1 which, with respect to the chemicalcomposition, are intended to be part of this disclosure.

On the MCrAlX, there may furthermore be a thermal barrier layer whichconsists for example of ZrO₂, Y₂O₃—ZrO₂, i.e. it is not stabilized or ispartially or fully stabilized by yttrium oxide and/or calcium oxideand/or magnesium oxide.

Rod-shaped grains are produced in the thermal barrier layer by suitablecoating methods, for example electron beam deposition (EB-PVD).

The guide vane 130 comprises a guide vane root (not shown here) facingthe inner housing 138 of the turbine 108, and a guide vane head lyingopposite the guide vane root. The guide vane head faces the rotor 103and is fixed on a fastening ring 140 of the stator 143.

1-35. (canceled)
 36. An alloy comprising in wt %: silicon 2.0%-4.5%;cobalt 0.5%-5%; carbon 2.0%-4.5%; molybdenum ≦1.5%; manganese ≦0.25%;nickel ≦0.5%; and iron.


37. The alloy as claimed in claim 36, wherein the proportion of silicon,cobalt and molybdenum is less than 7.5 wt %.
 38. The alloy as claimed inclaim 36, comprising from 1.0 wt % to 2.0 wt % cobalt.
 39. The alloy asclaimed in claim 36, further comprising molybdenum.
 40. The alloy asclaimed in claim 36, free of molybdenum.
 41. The alloy as claimed inclaim 36, further comprising manganese.
 42. The alloy as claimed inclaim 41, wherein the manganese content is ≦0.07 wt %.
 43. The alloy asclaimed in claim 36, free of manganese.
 44. The alloy as claimed inclaim 36, further comprising nickel.
 45. The alloy as claimed in claim36, free of nickel.
 46. The alloy as claimed in claim 36, furthercomprising 2.0 wt %-3.0 wt % silicon.
 47. The alloy as claimed in claim36, further comprising from 3.5 wt % to 4.0 wt % carbon.
 48. The alloyas claimed in claim 36, further comprising at most 0.07 wt % phosphorus.49. The alloy as claimed in claim 36, further comprising at most 0.008wt % sulfur.
 50. The alloy as claimed in claim 36, further comprising atmost 0.05 wt % magnesium.
 51. The alloy as claimed in claim 36, free ofchromium.
 52. The alloy as claimed in claim 36, free of magnesium. 53.The alloy as claimed in claim 36, consisting of iron, silicon, cobaltand carbon.
 54. The alloy as claimed in claim 36, consisting of iron,silicon, cobalt, carbon and manganese.
 55. A housing part, comprising analloy having iron and in wt %: silicon 2.0%-4.5%; cobalt 0.5%-5%; carbon2.0%-4.5%; molybdenum ≦1.5%; manganese ≦0.25%; nickel ≦0.5%.


56. A component, comprising an alloy having iron and in wt %: silicon2.0%-4.5%; cobalt 0.5%-5%; carbon 2.0%-4.5%; molybdenum ≦1.5%; manganese≦0.25%; nickel ≦0.5%,

wherein the component is a steam turbine or a gas turbine.
 57. Thecomponent as claimed in claim 56, a substrate which is iron-based orsteel-based.